After reading these interviews and seeing how Igarashi left on quite amicable terms, and how he was pleased with Lords of Shadow as a series I don't think he'll try to copy Castlevania or step on their vampire mythos toes just out if respect for Konami and the series.
He's quite grateful to them and wants the series to continue to be successful so I don't see him trying to undermine or sabotage Konami.
He still might step into the vampire mythos. There are a lot of vampire stories in the world that doesn't involve the European vampire model. And if I can recall correctly in an interview he gave years back, he is well aware of other vampire stories like the ones in Southeast Asia.
Yes, it was a good thing that he left Konami in good terms.
I would so be ok with IGA's first project being a side scrolling 3D Metroidvania if it looks this impressive.
As said in one interview, IGA might not immediately go into 3D. He would go there but not in the immediate future.
Here's an article that might be interesting for those who are wondering what the future might hold for our beloved series.
Well as if we haven't thought of some of the things listed there too.
http://www.hardcoregamer.com/2014/03/19/future-of-castlevania-in-limbo/78550/